Measles can cause immune amnesia

https://www.nationalgeographic.com/science/2019/03/measles-vaccine-protect-disease-immune-amnesia/

October 31, 2019

Measles is alarmingly contagious: It’s spread through the air and can linger in a room for up to two hours after an infected person has left. The disease is responsible for more than a hundred thousand deaths each year among unvaccinated people.

What’s more, danger still lurks even for those who survive an outbreak.

Measles not only weakens your immune system in the short term, bouts with the virus seem to wipe your immune system's memory, causing the body to forget how to fight off things that you may have already conquered. For some people, this so-called immune amnesia may linger for months to years after an infection.

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Before the vaccine was introduced in 1963, it was nearly guaranteed that every child would catch measles, and an estimated 2.6 million died from the disease each year. The measles vaccine, which is often combined with mumps and rubella, is incredibly effective, with two doses imparting nearly 97-percent protection, resulting in dramatic declines in measles deaths around the world.

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“The benefit of the vaccines has actually been kind of its own downfall,” says Yvonne Maldonado, an epidemiologist at Stanford Medical School. “People just don’t see this as a relevant intervention.”

However, the data show that’s not true: Not only did measles cases plummet once vaccine use became widespread, but cases of other diseases dropped as well—pneumococcus, diarrhea, and more. In resource-poor regions, the decline was as dramatic as 50 percent; in impoverished regions, it dropped by as much as 90 percent.

“We actually saw the whole overall baseline for childhood mortality drop precipitously,” says Harvard's Michael Mina, an author on a 2015 study analyzing this decline and lead author of the new study in Science. In essence, the measles vaccine seems to not only protect populations against measles, it may be keeping a slew of other infections at bay, and one way it could be doing this is through prevention of immune amnesia.

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Everywhere the monkey has lymphoid tissues—which house immune cells—glowing green speckles appeared, twinkling like stars in the night sky. These winks of light revealed that the virus favors what's known as immune memory cells. These cells record and catalog your lifetime of infections, helping you efficiently battle future invaders on repeat encounters. Later in the infection, the virus also settles into the surface lining of the lungs and nose, which help it launch into the air with each cough.

Once the immune system kicked in to clear the infection, this viral constellation vanished. The sudden darkness highlighted an important step of immune amnesia: By infecting memory cells, the virus not only wiped out some of the body's immune recollections, it essentially turned the system against itself by forcing healthy immune cells to kill off their infected comrades.



“If the virus didn’t kill those memory cells already, the immune system would finish the job off,” de Swart says. “We could literally see it happening under our hands.”

The team later confirmed that a similar mechanism is likely at play in humans. But it's unclear exactly how much immune memory vanished; that probably varies from person to person, de Swart notes. Still, the process emphasizes why secondary disease is so common with measles: Not only does the virus assault the immune system's first line of defense and damage your skin, respiratory, and gastrointestinal tract, it also erases your other hard-won resistances.

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So could secondary infections affect people who currently have measles in the United States?

“I absolutely think so,” Mina says, though he emphasizes that the U.S. healthcare system is strong enough to treat these infections and prevent most of them from turning deadly.

That isn't the case in many developing regions, where something as seemingly mild as diarrhea can kill. And because the virus attacks the immune system, the severity of infections depends on the general health of the population. In essence, measles acts like an infection amplifier, turning up the volume of background disease. This means that in resource-limited areas, the virus will cause higher overall death rates.

“In lots of developing countries still today, we see one in 50 or one in a hundred kids still dying from measles, primarily due to these sort of immune effects,” Mina says. To make matters worse, measles is contagious long before those infected even know it, making it easy for the virus to tag along if its unwitting hosts take to traveling.

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